1. Field of the Invention
The invention relates to a predistortion arrangement for a digital transmission system which transmits complex input data at the rate of a symbol clock H having a period T, with the aid of a modulator and a power amplifier which distorts the data, the arrangement comprising a predistortion circuit predistorting in opposite sense the input data before they pass through the amplifier in order to transmit the expected input data. The invention finds its applications in digital transmission systems such as data transmission modems, microwave links, space communications systems.
2. Description of the Related Art
For an efficient use of the available spectrum, the current digital transmission systems, more specifically, the microwave radio relay links and systems for data transmission over the telephone channel, use multilevel phase and amplitude modulation methods. These modulation methods are very sensitive to all sorts of distortion and, more specifically, to non-linear distortions from amplifiers, mixers and other non-linear circuits in the transmission chain. A particularly critical point with respect to microwave radio relay links and satellite transmission is the non-linearity of the send power amplifier or of the power amplifier on board in the case of satellite transmission. These amplifiers are known for their non-linear characteristics. If they are used in their linear zones their power is not utilized to full extent. If they are allowed to operate close to their saturation power levels, they will distort the signal in an unacceptable manner. In practice, with a given power amplifier, the level of the transmitted signal is fixed in order to realize a compromise between the signal-to-noise ratio and the non-linear distortion which the signal has undergone. Thus, the optimum operating point of the amplifier is that at which the joint effects of the additive noise of the channel and the non-linear distortion of the amplifier are minimized. For multilevel modulations (64-QAM and 256-QAM, for example) this point is remote from the saturation power of the amplifier, which means that the amplifier is not used in an efficient way. In order to augment its efficiency, predistortion techniques (fixed or adaptive) are currently used with which it is possible to reduce the effect the non-linearity of the power amplifier has on the transmitted signal.
A predistortion technique currently used consists of inserting into the intermediate-frequency stage of the transmitter a non-linear circuit approaching the inverse function of the power amplifier whose non-linearities are intended to be compensated. If the exact inverse of the function of the amplifier could be synthesized, this technique would make it possible to have a perfect output signal (without any non-linear distortion). However, this cannot be realized because a circuit of infinite complexity would be necessary for the exact inverse. In practice, it is sufficient to make an approximation and more often than not the Taylor series representing the non-linear function of the amplifier is restricted to the third order and a predistortion circuit is synthesized, also of the third order, so that the two cascaded circuits no longer have a third order distortion. Terms of higher order (5th and 7th order) appear at the output but have a low amplitude compared to the initial third order distortion. As a result, there is a certain improvement of the system performance. A disadvantage of these predistortion circuits in the intermediate frequency stage resides in the fact that they are analog circuits. They are hard to render adaptive and require intermittent interventions to readjust them and compensate for the time and temperature variations of the amplifier response. This predistortion technique is furthermore to be excluded if one wishes to have an automatic send power control.
Another more recent predistortion technique consists of modifying the alphabet of the data to be transmitted. This technique termed "data predistortion" or "baseband predistortion" is known from U.S. Pat. No. 4,291,277 as well as the article by A. A. M. SALEH and J. SALZ entitled "Adaptive Linearization of Power Amplifiers in Digital Radio Systems", Bell System Technical Journal, Vol. 62, April 1983, pp. 1019-1033.
In the article by A. A. M. SALEH and J. SALZ, FIG. 1 is a diagram of an adaptive predistortion circuit which produces a distorted constellation at the input of the modulator on the basis of the original square constellation, for example that of an amplitude modulation of two quadrature carriers (QAM). The amplifier affects the constellation by producing a net compression and a net rotation of the points having a large amplitude. In order to compensate for this effect the original constellation is distorted so that it resumes its original square form after passing through the power amplifier. Thus, when the predistortion circuit is optimized, it forms the reverse of the power amplifier (apart from gain and phase) and permits perfect compensation for the non-linearities of the amplifier. In order to make this circuit adaptive, the signal is taken off at the output of the amplifier, demodulated, then sampled at the transmit rate of the symbols 1/T and these samples are compared to the corresponding points of the QAM constellation used. These comparisons make it possible to obtain a control signal allowing of optimizing the predistortion circuit with the aid of a conventional algorithm. However, the diagram used in this FIG. 1 is very simplistic because it does not have any filtering before the modulator, or before the power amplifier. Thus it does not correspond to the solution generally used. In effect, in present systems one always uses a spectral shaping filter of the Nyquist type which makes it possible to limit the bandwidth of the signal while guaranteeing zero intersymbol interference at the decision instants. This filtering is generally equally divided between the transmitter and the receiver in order to maximize the signal-to-noise ratio at the decision instants. In systems of this type, the effect of the non-linearity of the amplifier is twofold: the constellation is not only deformed but intersymbol interference appears which associates a cloud of points to each point of the constellation. But the predistortion technique described hereinbefore does not make it possible to compensate this second effect.